The present invention relates to combustion systems. More specifically, the invention relates to an apparatus for detecting and monitoring the flame in a combustion system.
There is an increasing need for improved apparatus for accurately detecting the flame, and for performing diagnostic monitoring of the flame in combustors such as are found in land and aircraft gas turbine engines, industrial boilers, and other such machines. The combustion systems in these types of machines can be annular, tubular, can-annular, and other designs known to those skilled in the art. Accurate combustor flame detection and monitoring is necessary to prevent failure of the combustor during operation. In complex combustion systems, such as those used in gas turbine engines, flame instability can produce either a flashback or a flameout condition, either of which can lead to a catastrophic failure of the entire engine. Combustor failure can be prevented by ensuring that the combustor flame (or flames) is lighted during operation of the engine, by ensuring that the flame remains lighted continuously throughout the operation of the engine, and by ensuring that the flame remains stable, to prevent flashback, flameout, or any other combustion anomaly.
New demands presently being imposed on combustion systems, to meet increasingly stringent air pollution standards, require tighter control of the parameters at which combustors operate. While emission levels, especially of nitrogen oxides (NO.sub.x), carbon monoxide (CO), and unburned hydrocarbons (UHC) continue to be limited, the same or better power and performance is expected from combustor systems. These requirements have resulted in a development of low emission combustors which operate near the lean burn limit for reduced production of hazardous emissions. Many methods are used in combustion systems to reduce emissions, including variable geometry combustion system designs, lean pre-mixed designs, and staged combustion designs, among others. Operating combustors of these designs can sometimes lead to combustion instability, which may produce flashback or flameout conditions. Flashback occurs in premixed systems when the flame front propagates rapidly upstream from the steady state combustion zone. The upstream propagation of the flame can lead to significant damage if the flame reaches the area of the fuel injectors, and fuel flow cannot be disconnected or discontinued fast enough. Flameout conditions can occur under many different circumstances. Combustion instability, decreased equivalence ratio, and poor mixing (among other conditions) can lead to the downstream movement of the flame, out of the steady state combustion zone. This can lead to the flame actually being extinguished. In order to monitor the stability and condition of the flame, real-time combustion diagnostic systems which are capable of determining flameout and flashback conditions have been developed.
The most commonly used flame detector sensor for gas turbine combustors is the Geiger-Muller phototube. The phototube consists of a sealed glass envelope that contains a gas at a low pressure that is easily ionized. Two electrodes extend into the envelope and are separated by a short distance. During operation of the turbine engine, a high voltage potential is applied across the electrodes. When ultraviolet photons in the 180 to 260 nanometer range are emitted from the combustor flame and impinge on the tube, the gas within the envelope is ionized. The ionization process allows a current to flow between the electrodes producing a pulsed output. Signal conditioning circuitry, used in conjunction with the phototube, determines the pulse frequency of the output which is sent to a control system. A threshold frequency is set in the system indicating the presence of flame. When the frequency drops below the threshold, the system receives a signal corresponding to a loss of flame.
While Geiger-Muller tubes have been useful in monitoring flameout conditions, they have not been useful for detecting flashback because of their large size due to high voltage insulation, lack of viewing area discrimination and the difficulty of installing the tubes and their associated electronics into a complex burner design.
Geiger-Muller tubes can typically respond to a flame on/flame out condition in about 100 to 200 milliseconds (ms). For modern gas turbine engines and related applications operating with gaseous fuel, 100 to 200 ms is considered too slow to effectively signal the appropriate control values to stop the flow of fuel to the combustor, and thereby too slow to prevent damage to the engine. Geiger-Muller phototubes also typically operate at very high voltage levels (above 300 volts) which require special power supplies and can be dangerous to personnel working around the combustion system being monitored.
The most commonly used flashback sensor for gas turbine combustors are thermocouple based sensor systems. For flashback conditions, the thermocouple sensors respond too slowly; thermocouple sensors have historically been used for monitoring the occurrence of flashback in premixed combustion systems. Numerous thermocouple sensors are attached to the internal wall of the combustor in the mixing region, upstream of the combustion chamber. A control system monitors the thermocouples and specifically looks for sharp temperature rises that are indicative of a flashback condition. However, thermocouples are relatively slow to react and can be damaged easily under exposure to excessive temperatures. Because thermocouples are capable of measuring only local temperatures, a significant number of thermocouples are required to provide an effective flashback detection system in all areas of the combustion system. Further, if one or more thermocouples becomes damaged during operation of the engine, it is difficult, time consuming, and costly to repair the overall thermocouple sensing system.
Accordingly, the combustor system industry has a need for an apparatus for detecting both flameout and flashback conditions that is easy to install, provides a fast time response, and minimizes the number of installation points in the combustion system.